In recent years, portable wireless communication terminal devices have been increasingly used as means for mobile communications. As suitable antennas for these terminal devices, so called built-in antenna put inside the terminal device has been used more often. The terminal devices having the built-in antenna have advantages over generally accepted conventional ones having external antenna mounted on an exterior thereof in that the terminal devices having the built-in antenna are less likely to be damaged if dropped and provide a better freedom in the design of the portable wireless communication terminal device.
FIG. 1 is a perspective view of an exemplary planar inverted-F type antenna 65. The planar inverted-F type antenna 65 of FIG. 1 is a typical built-in antenna for use in a portable wireless communication terminal device. This antenna 65 comprises a radiative conductor section 66, a ground 67 opposing the radiative conductor section 66, a short-circuiting section 68, and a feeding pin 69, and thus, their appearance represent an inverted-F shape, as viewed from a side thereof. In this antenna 65, the resonance frequency thereof is determined by a size of the radiative conductor section 66.
FIG. 2A is a diagram for showing a current path in an antenna 65′ having the radiative conductor section with a slit therein. FIG. 2B is a diagram for showing a current path in an antenna 65″ having the radiative conductor section with a slot therein. The resonance length of each of the antennas 65′ and 65″ is made longer by forming a slit (cut out portion) 70 or a slot 71 in the radiative conductor section 66, as shown in FIGS. 2A and 2B. This allows the antennas 65′ and 65″ to be made more compact.
In FIGS. 2A and 2B, wave lines illustrate current paths in the respective antennas 65′ and 65″ before the slit 70 and slot 71 have been formed, that is, the antennas have been miniaturized. Solid lines illustrate the current paths after the slit 70 and the slot 71 have been formed to miniaturize the antennas 65′ and 65″. As shown in FIGS. 2A and 2B, the distance between the radiative conductor section 66 and the ground 67 (not shown in these figures) affects the bandwidth of an antenna like 65′ and 65″. For example, if the distance is made larger, the volume of a space formed between the radiative conductor section 66 and ground 67 increases, which in turn increases the bandwidth more often.
It is also possible to miniaturize the antenna 65 for example by filling the space between the radiative conductor section 66 and ground 67 with a dielectric material. In this case, however, the resultant bandwidth will be decreased. The short-circuiting section 68 is peculiar to a planar inverted-F type antenna structure, which is equivalent to a reduction in the radiating area of a planar antenna, typically a micro-strip antenna, having no short-circuiting section 68 to about ¼. The reduction is estimated according to a case where they are compared as their radiative conductor sections are respectively shaped into a square.
The feeding pin 69 is mounted at an appropriate position so that the input impedance of the radiative conductor section 66 matches the impedance of the feeding circuit formed on the circuit board. Mounting the feeding pin 69 at that position allows the antenna 65 or the like to be powered.
It is noted that on account of rapid popularization of cellular phones in recent years, only lines currently allocated for a single wireless communication system in a bandwidth are not sufficient for the phones. In order to secure sufficient lines, use of different bandwidths belonging to two kinds of wireless communication systems has been proposed. For example, there has been developed a portable wireless communication terminal device for dual-band, which enables use of two kinds of wireless communication systems in one portable wireless communication device.
Naturally, the antenna 65 and the like to be mounted on such dual-band terminal device are required to support the dual-band (i.e. dual frequency band) communication. This requirement for supporting the dual-band communication can be generally attained in a planar inverted-F type antenna by forming a slit 72 in the radiative conductor section 66 so as to establish two resonance lengths therein, as exemplified by a planar inverted-F type antenna 65′″ shown in FIG. 3. In FIG. 3, a longer current path f1 indicates the one relating to a low-frequency band, while a shorter current path f2 indicates the one relating to a high-frequency band.
FIG. 4A is a perspective view of a configuration of a portable wireless communication terminal device equipped with a planar inverted-F type antenna mounted on a circuit board 76 of the device, and FIG.4B is a side view thereof. Mounted at one position of the circuit board 76 of FIG. 4A is a planar inverted-F type antenna 65′. The portions of the antenna 65′ are shown schematically in FIGS. 4A and 4B. Elements not relevant to the antenna 65′ are omitted in the Figures.
In general, a portable wireless communication terminal device has the circuit board 76, a shield case (not shown) and a casing. The circuit board 76 includes an electronic circuit for operating the portable wireless communication terminal device. The shield case is provided for shielding the circuit board 76. The casing is provided for protecting the circuit board and the shield case.
When an antenna is built in the portable wireless communication terminal device of this type, the ground (not shown) of the circuit board 76 is formed as the ground 67 of the antenna 65′ and the like, the shield case is formed as the ground 67 or a shield case is formed in a part of an interior of the antenna 65′ and the like, as an intermediate therebetween. In FIGS. 4A and 4B, the simple structured circuit board 76 is illustrated as a ground 67 of the antenna 65′ and the like. In the example shown herein, the planar inverted-F type antenna of FIG. 2A is employed and the circuit board 76 is illustrated as the ground 67 of the antenna 65′.
In these portable wireless communication terminal devices having a built-in antenna 65 and the like, the casing is in general made of a non-conductive material such as a resin, at least in the neighborhood of the antenna. The radiative conductor section 66 is made of a sheet metal, which is glued to the inside of a non-conductive casing, or mounted on non-metallic (e.g. a resin) spacers provided between the radiative conductor section and the ground.
The short-circuiting section 68 and the feeding pin 69 are constituted of expandable spring connectors (feeding springs). The spring connectors are soldered to the circuit board 76 for mechanical and electrical coupling with the circuit board 76. However, the spring connector serving as the short-circuiting section is coupled to the ground of the circuit board, and the spring connector serving as the feeding element is connected to the conductive pattern of the circuit board, which is connected to the feeding circuit.
The circuit board 76 is in general loosely secured to the casing in order to reduce a strong shock that might damage the circuit board 76 when, for example, the portable wireless communication terminal device is dropped on the floor. Some portable wireless communication terminal devices utilize an inverted-L type antenna structure that is structurally equivalent to a monopole antenna bent at an intermediate point to miniaturize the antenna.
FIG. 5 shows a ¼-wavelength monopole antenna 80 for a given frequency. This antenna comprises an antenna element 79 put up on a vast ground plane 78 having a length longer than one wavelength. An image current can be established in the vast ground 78. Hence, the overall antenna characteristics of the antenna 80 is substantially the same as that of a ½-wavelength dipole antenna 82 consisting of symmetric antenna elements 81 and 81 as shown in FIG. 6.
FIG. 7 shows an inverted-L type antenna 85, i.e. antenna having an inverted-L shape as viewed from a side thereof. This antenna is equivalent to a monopole antenna bent midway, which can have a low profile. As an example, the inverted-L type antenna 85 has a ground 83 and an antenna element 84 mounted oh the ground. The antenna element 84 is formed by bending a monopole antenna at an intermediate point thereof.
However, a current passing through the horizontal section (the section parallel to the ground 83) of the antenna element 84 of the inverted-L type antenna 85 has an opposite phase as compared with the image current. As a result, the horizontal section does not greatly contribute to radiation, and thus the inverted-L type antenna 85 has a smaller radiation resistance than the ¼-wavelength monopole antenna has. Consequently, a length of the vertical section of the antenna element determines real part of the input impedance of the inverted-L type antenna, so that the real part is small.
On the other hand, a length of the horizontal section of the antenna element 84 determines the reactance part (i.e. imaginary part of the input impedance), which can be set to either capacitive large value or inductive large value depending on an electric length of the antenna element 84. Hence, it is not easy to match the impedance at the feeding point with only a general feed line of 50 Ω (Ohms). Inserting a matching circuit 89 between the antenna 87 and the ground 88 allows this problem to be solved.
For example, when the antenna impedance has a capacitive large value, an inductive reactance element (inductor) 91 is connected between the antenna 87 and the ground 88 so as to be parallel to them as shown in FIG. 8B. When the antenna impedance has an inductive large value, a capacitive reactance element (capacitor) 92 is connected between the antenna 87 and the ground 88 so as to be parallel to them as shown in FIG. 8C. Thus, this connection allows the impedance matching to be easily established in a simple circuit arrangement.
A inverted-L type antenna for dual-band (dual frequency band) can be implemented by setting two inverted-L type antenna elements for the respective corresponding bandwidths with the portions of the two antenna elements being sufficiently separated near their feeding points, thereby unaffected by their coupling.
Matching circuit for the dual-band antenna can be implemented in a simple form, similar to the one as shown in FIGS. 8A–8C. This is because it is possible to adjust the resonance lengths and the impedance of high-frequency and low-frequency inverted-L type antenna elements independently if they are so arranged as to be unaffected by their coupling. For this purpose, prior to inserting the matching circuit, the high-frequency impedance and low-frequency impedance are adjusted so that they occupy the same position on the Smith chart as much as possible. This adjustment can be done easily, for the reason mentioned above.
Each of the FIGS. 9A, 10A, and 11A is perspective view of a configuration of a portable wireless communication terminal device equipped with various types of planar inverted-L type antennas 96. Each of the FIGS. 9B, 10B, and 11B is schematic side view of the device. Irrelevant portions of the antennas 96 and 101 are omitted from any of these figures.
FIG. 9A is schematic diagram for showing a planar inverted-L type antenna 96 for a single-band and FIG. 9B is a side view thereof. An antenna element 97 of FIG. 9A is a radiative conductor section of the antenna 96, and then the element constitutes an inverted-L type antenna structure with a monopole antenna being bent to make it short. The antenna element 97 receives power from circuit board 98 via a feeding pin 99.
FIG. 10A is schematic diagram for showing a planar inverted-L type antenna 101 for dual-band and FIG. 10B is a side view thereof. Antenna elements 102a and 102b of FIG. 10A are radiative conductor sections of the antenna 101, and the elements constitute inverted-L type antenna structures with a monopole antenna being bent to make it short. The antenna elements 102a and 102b are fed power from circuit board 103 via a feeding pin 104. The radiative conductor section 102a having a shorter electric length for a high-frequency band is provided on a side of the feeding pin 104. The radiative conductor section 102b having a longer electric length for a low-frequency band is provided on the other side of the feeding pin 104.
FIG. 11 A schematic diagram for showing a configuration of a planar inverted-L type antenna 106 for dual-band and FIG. 11B is a side view thereof. Antenna elements 107a and 107b of FIG. 11A are radiative conductor sections of the antenna 106, each constituting an inverted-L type antenna structure with a monopole antenna being bent to male it short. The antenna elements 107a and 107b are fed power from circuit board 108 via respective feeding pins 109a and 109b. The built-in antennas as described above are usually disposed on the upper end of the back panel, and behind a speaker, of the portable wireless communication terminal device.
FIG. 12 is sectional side view showing a configuration of a portable wireless communication terminal device 111. The portable wireless communication terminal device 111 of FIG. 12 has a casing 112. Installed inside the casing 112 is a circuit board 113. A speaker 114 for allowing the voice of a caller to be output as sound, an LCD for displaying different kinds of information and the like are mounted on top of a surface of the circuit board 113.
Mounted on top of the backside, behind the speaker 114, of the circuit board 113, is a built-in antenna 115. Such the configuration that the built-in antenna 115 is disposed on top of the backside, behind the speaker 114, of the portable wireless communication terminal device 111 is due to a fact that the built-in antenna 115 can have a wider frequency band if it is disposed on top of the portable wireless communication terminal device 111, the antenna is least susceptible to a human body when the portable wireless communication terminal device 111 is called in progress, or the like.
When the built-in antenna 115 is installed inside the portable wireless communication terminal device 111, it is likely that a radiation characteristic of the built-in antenna 115 lessens if the device 111 is placed on a table T with its LCD and key board oriented upward as shown in FIG. 13, during its standby state, for example.
A cellular phone, which is an example of the portable wireless communication terminal device, has been more often used in recent years with it placed on a table because it is used not only as a mere voice communication tool but also as a data communications tool. Furthermore, if the table T is a metallic one, placing the device on the table T with the face of the LCD, etc., oriented upward results in a serious lessening of the antenna characteristic of the built-in antenna 115.
The antenna characteristic also lessens if fingers are placed over the built-in antenna 115 as shown in FIG. 14. It can be said that likelihood of covering the built-in antenna 115 with, for example, fingers is increasing in view of the fact that the portable wireless communication terminal device has been more and more miniaturized in recent years, even if the built-in antenna 115 is disposed in the above mentioned antenna arranging position, which is less susceptible to the human body.